Many types of multi-speed transmissions are available for motor vehicles. One such type is a six-speed planetary transmission having clutch-to-clutch shifting controls, as disclosed by Polak in U.S. Pat. No. 4,070,927. A higher number of forward speed ratios may be desirable to increase the operating range of the vehicle engine, improve shift quality, improve fuel economy, or for other reasons. For example, seven- and eight-speed automatic transmissions have now been developed. A number of potential challenges to commercial availability of higher-order transmissions exist; however, including increased size, complexity and cost of such transmissions.
The clutches or other shift mechanisms of automatic transmissions are typically controlled by electro-hydraulic systems in which electronic controls selectively actuate hydraulic valves, which control the distribution of pressurized fluid to engage and disengage the shift mechanisms upon command Examples of prior electro-hydraulic control systems are disclosed in U.S. Pat. Nos. 4,827,806; 5,601,506; 5,616,093; 6,520,881; and 7,140,993, all of which are issued to Long et al.